1. Field of the Invention
The present invention relates to an electroluminescent element, and more particularly to an electroluminescent element which is comparatively inexpensive and easy to produce, and is capable of emitting a blue visible light to a red one in a wide range of wavelength.
2. Detailed Description of the Related Arts
An electroluminescent element (hereinafter referred to as an "EL element") has a high distinguishability because it emits light therefrom, and has a large impact-resistance because it is a complete solid element. Various EL elements employing inorganic or organic compounds as the emitting layer have heretofore been proposed and have been under investigation for their practical use. Also proposed are luminescent elements having a structure, in which an electroluminescence and a fluorescence using it as an excitation light are co-used, have been proposed, and the arts as below have been developed.
For example, in Japanese Patent Application Laid-Open Nos. 25195/1985, 170194/1985, and 51796/1986, proposed is an EL element using an emitting layer the mixture of an inorganic EL material emitting bluish green light (including ZnS:CaCl, ZnS:CuMn, and so forth) and rhodamine-based fluorescent dye, to obtain a white light. According to these propositions, however, the fluorescent material emitting light is difficult to replace, since said EL material and fluorescent dye are mixed. As the bluish green EL light as excitation light is always included, the resulting light becomes whitened, and said EL element is not proper as the element for changing color. In addition, it suffers from the problem that a light of pure white in chromaticity cannot be obtained.
In Japanese Patent Application Laid-Open No. 220597/1985, proposed is a combined use of an electroluminescent fluorescent material which emits light with a peak wavelength of 460 to 520 nm (an inorganic EL material using ZnS : CuCl, for instance) and a wavelength-changing fluorescent material which emits light with a peak wavelength of 590 to 610 nm (specifically, coumarin- or rhodamine-based compound) upon the application of DC current. In that proposition, three kinds of EL elements are mentioned, that is, a single emitting layer in which these two materials are mixed; a laminate of these two materials, to be interposed between the cathode and the anode; and an EL element wherein a wavelength-changing (or a wavelength-converting) material is laminated entirely outside the structure of EL element. In that art, however, an inorganic material of DC current application-type is used as the EL element, and the range of its wavelength is so limited that the emitted light cannot change into various colors.
European Patent Publication No. 0281381, further, discloses an EL element having a structure in which an organic EL material such as 8-hydroxy-quinoline as the host material and a wavelength-changing fluorescent material such as a coumarin-based compound as a guest material are used, and a emitting layer consisting of these two materials is laminated on the hole injection layer. By doping a fluorescent material in a trace amount (1 mol % or smaller) to the emitting layer comprising an organic EL material, a red light to a green light with a high brightness is emitted. Therein, the mechanism of changing wavelength is not merely a mechanism in which the light emitted from the host material is absorbed by the guest material to emit therefrom, but is the result of transportation of energy from the host material to the guest material (J. Appl. Phys. 65. 3610 (1989)). According to said mechanism, an emission in a high efficiency is obtained indeed, but the resulting light is restricted to red to green light with a longer wavelength, since the light emitted from the host material has the wavelength of the green-range. Moreover, as a very slight amount of the guest material must be doped into the host material, it is difficult to form the emitting layer. Replacement of the guest material is also difficult, and consequently, said mechanism has a disadvantage that the color of the light cannot be changed easily. Further, the host and the guest must be closely adhered (the guest must be dispersed into the host), since transfer of energy is utilized.
Japanese Patent Application Laid-Open No. 51781/1982 (Japanese Patent Publication No. 7635/1989) proposes an organic EL element wherein the emitting zone comprises two emitting materials and the second emitting material is capable of changing the wavelength of the light emitted from the first emitting material.
Specifically, in an example disclosed, polystyrene dispersion film of tetraphenylbutadiene is used as the first emitting material, and the deposition film of the perylene-based compound is used as the second emitting material. The first emitting material alone emits a blue light with a peak wavelength of 467 nm, and when the second emitting material is added thereto, the emitted light is shifted to a near infrared ray with a peak wavelength of 810 nm. However, the light after change is not visible, and the second emitting material emits by a transfer of energy from the first emitting material in an excited state. Accordingly, it is required to make the film thickness of the first and the second emitting materials as small as 1000 .ANG., respectively, to adhere these two materials closely. A further problem is that it is impossible to replace the fluorescent material part to change the color of light.